FIG. 10 shows a configuration of a conventional type of switching power supply unit. As shown in the figure, this switching power supply unit 1 converts an input voltage from an externally connected input power supply 21 to a desired voltage by switching ON/OFF a switching transistor (Q1) 11, filters the voltage with a choke coil (L1) 12 for filtering as well as with a capacitor (C1) 13 to be outputted to outside, and supplies a specified voltage to a load 22 such as a main body of an externally connected computer. Then, a feedback control is executed in this switching power supply unit 1 so that an output voltage is a constant value.
Namely, an error voltage detecting amplifier 14 and a pulse width modulator (PWM1) 15 are provided in this switching power supply unit 1, the error voltage detecting amplifier 14 generates an error signal according to a difference between an output voltage (Vout) from the switching power supply unit 1 and a preset reference voltage (Vref), and the pulse width modulator (PWM1) 15 generates a control signal having a pulse width corresponding to an ON time of the switching transistor 11 (according to the error signal as a modulation signal) and provides controls for turning ON/OFF the switching transistor 11 via a buffer 16. In the figure, designated at the reference numeral 17 is a diode for regeneration, at 18a, 18b an input terminal, and at 19a, 19b an output terminal respectively.
A switching power supply unit disclosed in Japanese Patent Laid-Open Publication No. HEI 2-131358 controls operations of a switching transistor by comparing binary information for an output voltage as a reference to binary information for an actual output voltage.
In the switching power supply unit based on the conventional technology, however, feedback control is executed to turn ON/OFF the switching transistor according to an actual output voltage from the power supply unit, and for this reason, for instance, when an actual output voltage is displaced from the reference voltage due to fluctuation of an output current as shown in FIG. 11, a long time is required for changing of a pulse width of a pulse signal controlling ON/OFF of the switching transistor according to the feedback control in accordance with the displacement, and also a long time is required for an actual output voltage to match the reference voltage, which indicates that controllability is not sufficient.
Especially, when a load device connected to a power supply unit comprises a CMOS circuit as is the case in recent years, a current flows in the load only when the CMOS circuit operates, so that an output current is largely changed, accordingly an output voltage is also largely fluctuated, and control of an output voltage by means of the feedback control does not effectively function.
Also, in the switching power supply unit having the configuration shown in FIG. 10, a phase is delayed by 180.degree. in a filter circuit comprising L1 and C1, and further a delay of a phase by the error detecting amplifier 14 is added thereto. To prevent occurrence of a phenomenon known as a feedback loop, which is generated if a gain is 0 dB or more at a point of time when the phase has been delayed by 180.degree., the high frequency characteristic of the error detecting amplifier 14 has to be lowered in the switching power supply unit having the configuration shown in FIG. 10, which makes it impossible to realize a rapid response. Also to maintain stability of the feedback loop, the gain of the error detecting amplifier 14 is lowered from a low frequency, and for this reason, fluctuation of an output voltage to a change of an input voltage as well as of an output current becomes in turn large, and to deal with the fluctuation, a large number of capacitors have to be provided in the output side of the power supply unit, which causes the unit to be upsized.